Planing boats are widely operated in a variety of water and weather conditions. A planing hull is a hull designed to climb toward the surface of the water as power is supplied, thus reducing the friction or drag and therefore increasing hull speed. In other words a planing hull can rise bodily in the water and ride over the wave created by its progress through the water. The weight of a planing hull is supported by hydrodynamic lift and it does not displace an equal weight of water.
Planing boats with rigid “deep V” hulls and inflatable collars provide a robust and potentially fast craft. A “deep V” hull is able to plane at speed, while slicing through waves, minimising slamming at the bow. However, a slipway or hard surface is required for launching such a craft. Also, it is very difficult and potentially dangerous to launch such a craft in high waves or strong winds. Such craft are unable to cross exposed sand or mud banks, may be impeded in fouled water, or may strike submerged objects in flood water. In addition, conventional marine engines rely on water cooling. The water intakes of such conventional marine engines commonly become blocked in fouled or flood water, reducing engine reliability and compromising speed of response and safety of the craft to which they are fitted.
Known small hovercraft can be fully amphibious and are be able to cross-exposed sand or mud banks. They are immune to the problems in fouled water, -or flood water noted above. Conventional hovercraft engines rely on air cooling (for example, engines may be direct air cooled, or liquid-cooled with an air radiator) and are not compromised in fouled or flood water. Small hovercraft have hulls designed to ride over a surface on an air cushion. However, if the height of waves or obstructions is similar or greater than the height of the air cushion, such hovercraft cease to operate correctly and instead become displacement craft. A displacement craft is one having a hull that plows through the water displacing a weight of water equal to its own weight even when more power is added. As a displacement craft moves through water, the water is parted at the bow and closes in again at the stern.
The hull shapes of known hovercraft are not suited for use by a displacement craft. While such hull shapes do work, the hovercraft bounce and slam dangerously in waves. Similarly, thrust from air expelled at the back of the craft is not suited for use as a means for propelling a displacement craft. Again, such thrust works but provides poor acceleration and/or control. This means that such hovercraft can only operate in good weather conditions. Also, small hovercraft are also vulnerable to strong winds, or gusts, because they have very little friction with the surface, and so may be blown off course quite readily. Craft that do not alter the direction that the thrust air is expelled can have poor maneuverability, because controls in the air stream provide a turning moment on the body of the craft, but the course of the craft alters only as the craft loses momentum in an original direction through friction with the surface.
Moreover, hovercraft that are small enough to be transported by road without dismantling either have small rigid hulls, which limit their load-carrying capacity, or have a “loop and segment” skirt system which extends the lift cushion to a greater width around the hull. However, conventional “loop and segment” skirt systems have a large bag (loop) that collapses when lift air is turned off. The collapsed bag then impedes rapid loading on to a trailer. Some hovercraft have thrust reversers which provide a braking force. However, it can be extremely difficult to steer the craft while operating the reverse thrust.
It has previously been proposed (U.S. Pat. No. 3,389,672) to provide a displacement hulled vessel capable of operating as a gaseous cushion borne craft. However, such a vehicle would suffer in use from the slamming at the bow noted above or from limited speed associated with the displacement hull.
U.S. Pat. No. 4,535,712 shows a vessel having a variable geometry hull capable also of operating on the air cushion principle. This presents the problem that the geometry of the hull must be altered before the vehicle can change its mode of operation.
It is an advantage of the present invention that it eliminates or at least substantially reduces the problems noted above, while maintaining the benefits of the different craft considered.